The present invention relates to a method for gluing two disc elements together, in particular substrates for digital audio, video or data discs, such as Digital Versatile Discs (DVDs), the method comprising the steps
a) the two disc elements are held essentially horizontally and coaxially with one another in such a manner that a gap is formed between the disc elements, and
b) in an inner area of the gap, a liquid adhesive is applied in the form of at least one strand in the circumferential direction on both the facing sides of the two disc elements.
A method of a similar kind is previously known from WO 97/36737. In this known method, a predetermined volume of the adhesive is applied in such a manner that it comes into contact essentially simultaneously with the facing side surfaces of the disc elements, when the adhesive flows out of the outlet of a gluing nozzle introduced into the gap, so that, when simultaneous turning or rotation of the disc elements through approximately one revolution is carried out, an annular adhesive strand is formed, the height of which fills the height of the disc gap, after which the discs are brought together, so that the adhesive, by means of its capillary forces and the weight of the upper disc element, spreads and fills the space between the disc elements to form a thin, homogeneous adhesive layer of predetermined thickness. In the manufacture of certain types of optical discs for data storage, there are extremely stringent requirements that no gas inclusions (air bubbles and the like) should be present in the adhesive layer between the disc elements. It is also desirable to perform the gluing process as rapidly as possible and in such a manner that no surplus adhesive forces its way out of the gap during joining together, which would necessitate an additional collecting stage.
According to the present invention, it has been found that these requirements can be met in a method of the kind referred to in the introduction, which is characterized in that
c) the adhesive is applied in separate, circular-arc-shaped strands with the same radius of curvature on the facing side surfaces of the disc elements and with a combined height of the adhesive strands which is greater than the height of the gap between the disc elements;
d) the disc elements are rotated in relation to one another through approximately one revolution after the application of the respective adhesive strand, one end of the adhesive strand on one disc surface being made to come together with one end of the adhesive strand on the other disc surface, after which
e) the disc elements are brought together against one another and made to rotate rapidly in conjunction with the bringing together of the disc elements in order, by means of the gravitational force of the upper disc element and the capillary force of the adhesive, in association with the centrifugal force on the latter during the rapid rotation, to form a homogeneous and bubble-free adhesive layer of predetermined thickness between the disc elements.
By virtue of the fact that one end of one adhesive strand on one disc surface is made to come together with one end of another adhesive strand on the other disc surface after application of the adhesive, a subsequent rotation of the disc elements in relation to one another causes the adhesive strands to be combined to form a single, homogeneous, air-bubble-free adhesive ring by virtue of a shearing action being exerted between the adhesive strands.